An RF communications system may comprise a transmitter and a receiver that communicate via a radio frequency (RF) channel. The transmitter may encode information in a symbol that is transmitted via the RF channel. The transmitter may utilize a modulation type to encode information into a symbol, s. The modulation type may comprise a plurality of constellation points that represent distinct combinations of binary bits. The transmitter may encode information comprising binary bits of information by selecting a modulation type, and within the selected modulation type, selecting a constellation point to represent the binary bits of information. A binary bit may also be referred to as a bit. The transmitter may generate signals corresponding to the constellation point that may comprise in-phase (I) and quadrature phase (Q) signals. The correlation between a constellation point and I and Q signals may comprise a mapping. The I and Q signals may be transmitted by the transmitter as an IQ signal via the RF channel.
The RF channel may distort the transmitted IQ signal from the transmitter such that, at the receiver, the received signals IR and QR may differ in magnitude and/or phase from the corresponding transmitted signals I and Q. In addition, the RF channel may introduce noise into the signal.
A task for a receiver in achieving successful reception of information, via the RF channel, from the transmitter may comprise a plurality of steps to determine, based on a received IRQR signal, the binary bits, of information that were transmitted by the transmitter. One step may comprise detecting a symbol from the received IRQR signal. The receiver may utilize a modulation type to decode the IRQR signal. The receiver may utilize a corresponding modulation type to the modulation type utilized by the transmitter. The correlation between the signals IR and QR and a constellation point may comprise a demapping. Because the signals IR and QR at the receiver may differ from the corresponding signals I and Q at the transmitter, the receiver may be unable to correlate the signals IR and QR to a constellation point. The receiver may utilize various heuristics to demap the signals IR and QR to a constellation point. The selected constellation point may comprise an estimate, ŝ, of the symbol, s, that was transmitted by the transmitter.
Since the receiver utilizes an estimate of the transmitted signal, ŝ, there is a statistical probability that the constellation point associated with estimate ŝ may differ from the corresponding constellation point associated with original symbol, s. Consequently, there is a statistical probability that at least one binary bit of information retrieved from the estimate, ŝ, may differ from a corresponding binary bit of information in the original symbol, s, that was transmitted by the transmitter. Such a difference in one or more received binary bits of information may constitute a communications error between the transmitter and the receiver that may be measured by a packet error rate (PER) statistic.
The task of assessing the statistical probability that at least one binary bit of information in an estimate, ŝ, is equivalent to a corresponding binary bit of information in an original symbol, s, may comprise comparing the estimate ŝ to a plurality of constellation points in a constellation map. In some conventional approaches, the task of evaluating these comparisons may be of exponential complexity as the number of constellation points in a constellation map may increase exponentially with an increase in the number of binary bits contained in an original symbol, s. A comparison between the estimate ŝ and at least one constellation point in a constellation map may be referred to as a “search”. A comparison between the estimates and each of the plurality of constellation points in a constellation map may be referred to as a “full search”. The number of comparisons in a full search may increase exponentially with an increase in the number of bits contained in the original symbol, s.
A transmitter may also utilize a plurality of symbols to encode a plurality of binary bits of information. The transmitter may transmit the plurality of symbols concurrently. Each of the plurality of symbols may be transmitted by a corresponding plurality of transmitting antennas. Each of the plurality of symbols may be transmitted in a corresponding stream. The stream transmitted by each transmitting antenna may be referred to as a “layer”. At the receiver, the received plurality of symbols may be compared to constellation points in a constellation map in which the number of comparisons may also increase exponentially with an increase in the number of symbols in the transmitted plurality of symbols. Alternatively, each symbol in the received plurality of symbols may be associated with a corresponding constellation map for the symbol. Within a layer, a symbol in a received plurality of symbols may be compared to constellation points in a constellation map. Jointly comparing each symbol in the received plurality of symbols to constellation points in each corresponding constellation map may still result in an exponential increase in the number of comparisons with an increase in the number of symbols in the transmitted plurality of symbols.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.